1. Field of the Invention
This invention relates to a method for efficiently removing fine particles adhering to synthetic resin pellets by an electrostatic charge and an apparatus which can be used to practise the method. This invention further relates to a stock yard of a synthetic resin pellets.
2. Prior Art
In general, synthetic resin pellets carry fine particles which are generated during a step of producing the pellets or transportation of them. The presence of such fine particles causes a variety of troubles in a molding process with the use of the synthetic resin pellets, decreasing a commercial value of the synthetic resin pellets. Accordingly, the fine particles have to be removed as much as possible.
In recent years, synthetic resins, for example, polycarbonate resins have been used in an information recording substrate or all optical equipment such as optical lenses or the like. Since, in the optical usage, incorporation of a foreign matter has to be extremely excluded, the molding process of the optical equipment is generally carried out in a clean room. Accordingly, a permissible amount of the 16- mesh-pass fine particles carried with the polycarbonate resin pellets which are used in the optical equipment is considered to be 40 ppm or less.
Several methods for removing fine particles from synthetic resin pellets have been hitherto known. For example, there is a method in which fine particles are removed through a wire net by colliding synthetic resin pellets against the wire net through a high-speed air flow (15 m/second or more). Such method, however, causes a trouble that when the wire net is broken, a metal wire is incorporated into the synthetic resin pellets. When a punching metal is used instead of the wire net to avoid this trouble, problems such as crushing of the synthetic resin pellets tend to occur. There is also proposed a method in which fine particles are removed through a rotary drum covered with a wire net or a vibratory screen. In this method, high removal percentage of 9-mesh-pass and 16-mesh-on fine particles can be gained. According to the present inventors' actual measurement, it was 80 to 90%. However, removal percentage of the 16-mesh-pass fine particles is insufficient, and the present inventors' actual measurement showed that it was 60 to 75%. Further, there is a method in which synthetic resin pellets are caused to fall on a top of a conical body, and when the synthetic resin pellets fall from the peripheral portion of the conical body, the fine particles are removed by a rising air flow, and a method in which synthetic resin pellets are caused to fall while successively colliding against wall surfaces of a tower which is bent in a zigzag configuration wherein the fine particles are removed by a rising air flow. However, in these methods as well, removal percentage of 16-mesh-pass fine particles is insufficient, and according to the present inventors' actual measurement, it was 70 to 80%.
U.S. Pat. No. 4,631,124 discloses the kinetic gravity deduster employing gravity to feed the dust and impurity laden particulate material through a linear kinetic energy cell, which cell generates an electric field to neutralize the electrostatic charges causing the dust to adhere to the particulate material.
However, the above patent neither describes nor suggests the present invention.